The present invention generally relates to a multiplex control system and more particularly to the multiplex control system imparted with diagnosis function.
In general, the control system to which a great importance is put is implemented with redundancy by multiplexing the control in an effect to enhance the reliability of the control system as a whole. As a typical example of such control systems, there can be mentioned a nuclear reactor protecting system in a nuclear power plant in which control channels are provided in quadruple in order to ensure an extremely high reliability. In this connection, reference may be made, for example, to U.S. Pat. No. 4,517,154 and EP-A-180085 which are both concerned with such type of nuclear reactor protecting system as mentioned above. More specifically, in the nuclear reactor protecting system disclosed in these literatures, control systems or channels are provided in quadruple, wherein control signals outputted from these four control channels are inputted to a majority decision circuit configured in the form of a 2-out-of-4 logic voting circuit so that a device subject to control such as, for example, a valve is actuated on the basis of the control signal selected by the majority decision circuit. In the case of the nuclear reactor protecting system disclosed in the above-mentioned U.S. Patent, a diagnosis unit is provided for diagnosing the function of all the quadruplexed control channels. In more concrete, a single diagnosis unit produces test signals sequentially to the quadruplexed control channels for making decision as to the presence or absence of abnormality or malfunction in each of the control channels.
In the nuclear reactor protecting system of the structure mentioned above, occurrence of abnormality (malfunction) in the diagnosis unit itself might lead to such utterly undesirable situation in which all the quadruplexed control channels could simultaneously output erroneous test signals, resulting in that the device subject to the control might be inadvertently or erroneously operated. It is further noted that in this known nuclear reactor protecting system, the abnormality decision is not performed for the majority decision circuit, i.e. the signal selecting circuit itself.